Date: Tue, 16 Apr 2019 00:35:41 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Strange coding in mvdistinct.c
Message-ID: <20190415223541.63ayu2aztt4qqh3l@development>
References: <29785.1555365602@sss.pgh.pa.us>
 <30400.1555366344@sss.pgh.pa.us>
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On Mon, Apr 15, 2019 at 06:12:24PM -0400, Tom Lane wrote:
>Oh, and as I continue to grep, I found this in dependencies.c:
>
>            dependencies = (MVDependencies *) repalloc(dependencies,
>                                                       offsetof(MVDependencies, deps)
>                                                       + dependencies->ndeps * sizeof(MVDependency));
>
>I'm pretty sure this is an actual bug: the calculation should be
>
>                       offsetof(MVDependencies, deps)
>                       + dependencies->ndeps * sizeof(MVDependency *));
>
>because deps is an array of MVDependency* not MVDependency.
>
>This would lead to an overallocation not underallocation, and it's
>probably pretty harmless because ndeps can't get too large (I hope;
>if it could, this would have O(N^2) performance problems).  Still,
>you oughta fix it.
>
>(There's a similar calculation later in the file that gets it right.)
>

Thanks. I noticed some of the bugs while investigating the recent MCV
serialization, and I plan to fix them soon. This week, hopefully.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services